Dr. Karl Hempel said Monday that he was told Chiles' heart weighed 600 grams instead of the 350 grams typical for a human heart.

"I was kind of surprised that it was real enlarged," said Hempel, Chiles' physician for several years.

The enlargement, caused perhaps by Chiles' high blood pressure, could have contributed to the abnormal heart beat -- or arrhythmia -- that caused him to die, Hempel said.

He diagnosed Chiles' bronchitis Nov. 4 and listened to his heart. Chiles got a full cardiac workup in 1995 after he suffered a minor stroke, but showed no major problems, Hempel said.

But the arrhythmia that struck Chiles is more common than many realize -- and harder to predict.

It is the most common cause of sudden death, claiming more than 300,000 Americans each year. Half of all cardiac deaths happen that way. A person who has had previous heart disease is at greater risk, though some people have died of an arrhythmia with no knowledge that they had heart problems.

"It's basically an electrical storm of a heart in which the heart becomes completely uncoordinated," said Ronald J. Myerburg, director of the cardiology division at the University of Miami and president of the Cardiac Arrhythmia Research and Education Foundation.

Among its victims: A. Bartlett Giamatti, the former baseball commissioner and president of Yale University, who died in 1989 at the age of 51; and Jim Fixx, the famous jogger and bestselling author whose books made the sport popular, who died at 52.

Several sports stars have been diagnosed with minor forms of arrhythmia, such as basketball player Hakeem Olajuwon. Boston Celtics star Reggie Lewis died suddenly of an arrhythmia in 1993, and college basketball player Hank Gathers collapsed and died during a game because of a heart condition.

In Chiles' case, he was at risk because of previous heart disease and his high blood pressure, which was hereditary. He underwent major heart surgery in 1985, increasing the risk.

Still, Hempel noted that he outlived other relatives who died in their 50s of high blood pressure. Chiles was on medication, ate a low-fat diet rich in fruits and vegetables and was told to exercise 30 minutes a day, Hempel said.

The governor died while using a stationary exercise bike -- raising questions about whether he should have been exercising. But both Hempel and Myerburg said there was no indication Chiles should have worried about exercising.

In fact, Hempel thinks Chiles' commitment to staying fit helped him.

"I think the exercise probably prolonged his life," Hempel said. -- Times researcher John Martin contributed to this report.